Flex-6300 RX antenna switching not following TX Band

When I'm running two panadapters, say slice A on Panadaptor 1 for 80M and slice B on Panadaptor 2 for 40M, it does not seem possible to save a different default RX antenna for each band.

I appreciate that with a single SCU it is only possible to have one RX antenna in use at any time across both panadaptors and slices. But I would like to have the option of setting a different default RX antenna for each band and for this to be switched in when the relevant slice is activated for TX. Is this not possible, or have I overlooked a configuration setting somewhere?

I believe your observation is correct. That radio only has one SCU so one antenna is all you can get. The SCU digitizes the signals and can only hook to one antenna at a time. The 6600s and 6700s have two SCU but carry a much higher price point.